1a) If you happen to have Photoshop or another editor make sure you have a VTF plugin.
1b) Save your pic in .vtf format after resizing it to 256x256.
2a) If you don't have a good photo editor then you can always get VTF Edit which is Open Source:
http://nemesis.thewavelength.net/index.php?c=178#p178
2b) Import a JiF/JPG/etc. into VTFEdit. When you import it click on the resize option and select 256x256.
3) Save it here: C:\Program Files\Valve\Steam\steamapps\<YOURACCOUNTNAME>\team fortress 2\tf\materials\VGUI\logos
4) Copy it here: C:\Program Files\Valve\Steam\steamapps\<YOURACCOUNTNAME>\team fortress 2\tf\materials\VGUI\logos\UI
5) Start TF2 from Steam
6) In the main TF2 menu select options
7) Click on Multiplayer
Click on Import Spray9) Navigate to the logos directory where you put your .vtf file and import it
Voila. You now can annoy friends and neighbors by spraying your logo on their walls!